I'm building my little base so that I can see around where I'm currently located at night, so see whether it's safe to go out the next morning, etc. I'd like to make my outer wall out of glass for this reason.

In order to keep Creepers from being dangerous while viewing from inside the base, however, I'm planning a two walled base. The outer wall keeps creepers far enough away from the player (who is confined to the inner wall), such that so long as the player is anywhere inside the inner wall, the creeper cannot detonate.

In order to do that, I need to know how much space to leave between the outer and inner walls. And in order to do that, I need to know what the creeper's activation radius is.

Yes, creepers need line of sight to activate. However, all mobs will appear to "look" at you even if they don't have LoS; they just won't become aggressive (or start counting down, in the creeper's case).
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Doktor JJun 13 '11 at 5:36

You should only need two blocks between you and the creeper. They only activate if they are within one block of you. I created a two layer wall in my base and would just stare at them all day long right in front of me, they never activated. You might get away with one block but just to be safe and since I know two works, I would stick with that.

Have you experienced creepers blowing you up through glass before? I was under the impression that glass was as good as obsidian in terms of sight -- as far as mobs are concerned, anyway.
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Raven Dreamer♦Nov 13 '10 at 5:43

@Raven: No -- I just didn't want to take the chance. I didn't think the creeper needed line-of-sight in order to decide to blow up.
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Billy ONealNov 13 '10 at 17:19